Many polymers or blends of polymers are relatively non-conductive. As such, this can result in a static charge build-up during processing and use of the polymer. The charged polymer molded parts can attract dust, which are small particles, and can thus interfere with a smooth surface appearance. The attracted particles to the surface of a molded article may also cause a decrease in the transparency of the article. In addition, the electrostatic charge can be a serious obstacle in the production process of such polymers. In the past, electrically conductive agents such as carbon and metallic particles or surfactants were used in various attempts to reduce electrostatic charges of synthetic macromolecular materials by mixing them internally together or by coating the material with an agent. These methods employing electrically conductive agents are not generally feasible for many reasons such as the large amount of agents which must be usually used, the difficulty in adding them to the material, the difficulty in obtaining a transparent product or retention of mechanical and rheological properties, if that is the case, and the high cost of such conductive agents. Thus, these agents can be used only in limited situations.
Anti-static agents are materials which are added to polymers to reduce their tendency to acquire an electrostatic charge, or when a charge is present, these anti-static agents promote the dissipation of such a charge. The anti-static agents are usually hydrophilic or ionic in nature. When present on the surface of polymeric materials, they facilitate the transfer of electrons and thus eliminate the build up of a static charge. Anti-static agents have been applied in two ways. One method uses external anti-static agents that are applied by spraying the surface or dipping of the polymeric material. The second method uses internal anti-static agents, which are added to the polymer before processing. It is necessary for anti-static agents applied in this manner that they are thermally stable and able to migrate to the surface during processing.
Since there are many anti-static agents having surfactants as their main constituent, appropriate ones may be selected therefrom according to the situation. In fact, many of the types to be internally added have been considered and tried. When used as an internally-applied anti-static agent, however, anionic surfactants are difficult to handle because they are inferior in compatibility and uniform dispersibility and tend to decompose or deteriorate when heated. Cationic surfactants containing quarternary nitrogen in their molecules and amphoteric surfactants, on the other hand, can be used only in limited situations because they are extremely poor in heat resistance, although their anti-static characteristics are good. As for non-ionic surfactants, they are relatively superior to the aforementioned ionic surfactants regarding compatibility with synthetic macromolecular materials, but tend to be weak in anti-static characteristics and their effects disappear with time at normal or high temperatures. Moreover, because of the limited thermal stability of these non-ionic surfactant anti-static agents, their use with engineering thermoplastic resins, such as aromatic polycarbonates, is also limited due to the temperatures at which such resins are processed. Thus, these types of surfactants adversely affect the optical properties of aromatic polycarbonates. Although metal salts of organic sulfonic acids have been reported, especially as internally applied anti-static agents for polycarbonates and polyester resins which are molded at high temperatures, they are not sufficient in compatibility with resins or heat resistance one adverse consequence of insufficient compatibility is that transparency characteristics of certain macromolecular materials such as polycarbonates are lost with such anti-static agents. There has also been a report of using phosphonium salts or organic sulfonic acids having halogen substituent as a flame retardant (U.S. Pat. No. 4,093,589), but they are not to be expected to serve as anti-static agents as well.
Another patent discloses reducing the static charge on polycarbonate resins. This is U.S. Pat. No. 4,943,380, which discloses an anti-static composition containing 90-99.9 weight % of polycarbonate and 0.1-10 weight % of a heat resistant phosphonium sulfonate having the general formula: ##STR1##
where R is a straight or branched chain alkyl group having from 1 to 18 carbon atoms; R.sub.1, R.sub.2 and R.sub.3 are the same, each being an aliphatic hydrocarbon with 1-18 carbon atoms or an aromatic hydrocarbon group; and R.sub.4 is a hydrogen group with 1-18 carbon atoms. The corresponding cationic surfactants containing quarternary nitrogen in their molecules can only be used in limited situations, because they are extremely poor in heat resistance although their anti-static characteristics are good (U.S. Pat. No. 5,468,973).